2.1.2实例变量非线程安全

若多个线程共同访问1个对象中的实例变量，那么有可能出现非线程安全的问题。

测试如下

 1 package com.cky.bean;
 2 
 3 /**
 4  * Created by chenkaiyang on 2017/12/4.
 5  */
 6 public class SelfPrivateNum {
 7     int num =0;
 8     public void addI(String usrName) {
 9         try {
10             if (usrName.equals("a")) {
11                 num  =100;
12                 System.out.println("a over");
13                 Thread.sleep(2000);
14             } else {
15                 num= 200;
16                 System.out.println("b over");
17 
18             }
19             System.out.println(usrName + " num="+ num);
20         } catch (InterruptedException e) {
21             e.printStackTrace();
22         }
23     }
24 }

package com.cky.thread;

import com.cky.bean.SelfPrivateNum;

/**
 * Created by chenkaiyang on 2017/12/4.
 */
public class ThreadA  extends Thread{
    private SelfPrivateNum sn;
    public ThreadA (SelfPrivateNum sn) {
        this.sn = sn;
    }
    @Override
    public void run() {
        super.run();
        sn.addI("a");
    }
}

 1 package com.cky.thread;
 2 
 3 import com.cky.bean.SelfPrivateNum;
 4 
 5 /**
 6  * Created by chenkaiyang on 2017/12/4.
 7  */
 8 public class ThreadB extends Thread{
 9     private SelfPrivateNum sn;
10     public ThreadB (SelfPrivateNum sn) {
11         this.sn = sn;
12     }
13     @Override
14     public void run() {
15         super.run();
16         sn.addI("b");
17     }
18 }

package com.cky.test;

import com.cky.bean.SelfPrivateNum;
import com.cky.thread.ThreadA;
import com.cky.thread.ThreadB;

/**
 * Created by chenkaiyang on 2017/12/2.
 */
public class Test {
    public static void main(String[] args){
        SelfPrivateNum sn = new SelfPrivateNum();
        ThreadA threadA = new ThreadA(sn);
        ThreadB threadB = new ThreadB(sn);
        threadA.start();
        threadB.start();
    }
}

D:itjdk1.8injava -Didea.launcher.port=7533 "-Didea.launcher.bin.path=D:itideaIntelliJ IDEA 2016.3.3in" -Dfile.encoding=UTF-8 -classpath "D:itjdk1.8jrelibcharsets.jar;D:itjdk1.8jrelibdeploy.jar;D:itjdk1.8jrelibextaccess-bridge-64.jar;D:itjdk1.8jrelibextcldrdata.jar;D:itjdk1.8jrelibextdnsns.jar;D:itjdk1.8jrelibextjaccess.jar;D:itjdk1.8jrelibextjfxrt.jar;D:itjdk1.8jrelibextlocaledata.jar;D:itjdk1.8jrelibext
ashorn.jar;D:itjdk1.8jrelibextsunec.jar;D:itjdk1.8jrelibextsunjce_provider.jar;D:itjdk1.8jrelibextsunmscapi.jar;D:itjdk1.8jrelibextsunpkcs11.jar;D:itjdk1.8jrelibextzipfs.jar;D:itjdk1.8jrelibjavaws.jar;D:itjdk1.8jrelibjce.jar;D:itjdk1.8jrelibjfr.jar;D:itjdk1.8jrelibjfxswt.jar;D:itjdk1.8jrelibjsse.jar;D:itjdk1.8jrelibmanagement-agent.jar;D:itjdk1.8jrelibplugin.jar;D:itjdk1.8jrelib
esources.jar;D:itjdk1.8jrelib
t.jar;F:springboot	hreaddemooutproduction	hreaddemo;D:itideaIntelliJ IDEA 2016.3.3libidea_rt.jar" com.intellij.rt.execution.application.AppMain com.cky.test.Test
a over
b over
b num=200
a num=200

Process finished with exit code 0

结果分析：出现了非线程安全的问题，那如何解决呢？

使用synchronized关键字

 1 package com.cky.bean;
 2 
 3 /**
 4  * Created by chenkaiyang on 2017/12/4.
 5  */
 6 public class SelfPrivateNum {
 7     int num =0;
 8    synchronized public void addI(String usrName) {
 9         try {
10             if (usrName.equals("a")) {
11                 num  =100;
12                 System.out.println("a over");
13                 Thread.sleep(2000);
14             } else {
15                 num= 200;
16                 System.out.println("b over");
17 
18             }
19             System.out.println(usrName + " num="+ num);
20         } catch (InterruptedException e) {
21             e.printStackTrace();
22         }
23     }
24 }

D:itjdk1.8injava -Didea.launcher.port=7534 "-Didea.launcher.bin.path=D:itideaIntelliJ IDEA 2016.3.3in" -Dfile.encoding=UTF-8 -classpath "D:itjdk1.8jrelibcharsets.jar;D:itjdk1.8jrelibdeploy.jar;D:itjdk1.8jrelibextaccess-bridge-64.jar;D:itjdk1.8jrelibextcldrdata.jar;D:itjdk1.8jrelibextdnsns.jar;D:itjdk1.8jrelibextjaccess.jar;D:itjdk1.8jrelibextjfxrt.jar;D:itjdk1.8jrelibextlocaledata.jar;D:itjdk1.8jrelibext
ashorn.jar;D:itjdk1.8jrelibextsunec.jar;D:itjdk1.8jrelibextsunjce_provider.jar;D:itjdk1.8jrelibextsunmscapi.jar;D:itjdk1.8jrelibextsunpkcs11.jar;D:itjdk1.8jrelibextzipfs.jar;D:itjdk1.8jrelibjavaws.jar;D:itjdk1.8jrelibjce.jar;D:itjdk1.8jrelibjfr.jar;D:itjdk1.8jrelibjfxswt.jar;D:itjdk1.8jrelibjsse.jar;D:itjdk1.8jrelibmanagement-agent.jar;D:itjdk1.8jrelibplugin.jar;D:itjdk1.8jrelib
esources.jar;D:itjdk1.8jrelib
t.jar;F:springboot	hreaddemooutproduction	hreaddemo;D:itideaIntelliJ IDEA 2016.3.3libidea_rt.jar" com.intellij.rt.execution.application.AppMain com.cky.test.Test
a over
a num=100
b over
b num=200

Process finished with exit code 0

结果分析：使用synchronized关键字，当a拿到锁之后，只有当前执行完毕释放锁的时候，b才能拿到锁，解决了线程安全。而且代码的执行是同步的，因为只有1个对象。